Cleaning the air requires a substantial quantity of plants. In a 1989 NASA study, plants exposed to volatile organic compounds (VOCs), air pollution chemicals, demonstrated promising results. The combination of plants, soil, and soil microbes showed the potential to reduce VOC levels in the air. The excitement stemmed from the fact that plants, being more attractive, sustainable, and self-replicating, offered a more appealing solution than conventional air cleaning technologies.

However, upon closer examination of this study conducted over 30 years ago, it becomes apparent that the impact of indoor potted plants on air quality may have been overstated by the authors of the NASA study and a few subsequent researchers. Recent studies suggest that achieving a noticeable difference in air quality would necessitate an impractical number of houseplants.

Interestingly, it seems that indoor spaces require a similar plant density to outdoor environments for effective air purification. However, exceeding 20% visible vegetation indoors might be counterproductive, as it can be visually distracting or overwhelming. This raises questions about the efficiency of using plants as air purifiers indoors. To gain a better understanding, let's delve into current research on how plants interact with various pollutants and determine the quantity needed to effectively purify the air.

Do plants remove VOCs?

A notorious review article published in Nature in 2020, titled "Potted plants do not improve indoor air quality: a review and analysis of reported VOC removal efficiencies," conducted by Bryan Cummings and Michael Waring at Drexel University, scrutinized 196 different VOC removal tests across 12 published papers. Cummings and Waring highlighted several flaws in the methods employed in the reviewed papers, such as utilizing sterile sealed chambers barely accommodating the plant alone and introducing VOCs once, allowing the plant to sit with them. In reality, indoor plants coexist in large rooms with people, furniture, building materials, and ongoing sources of VOCs.

VOC, an acronym for volatile organic compounds, encompasses a broad range of substances that tend to become gases at room temperature. Intense odors from paints, glues, lacquers, and similar products indicate high concentrations of VOCs in the air, along with other potent smells like alcoholic beverages, essential oils, or new synthetic materials. Most households consistently harbor low levels of VOCs from items such as new clothing, furniture, building materials, or cleaning products. While VOCs generally do not cause immediate impacts, prolonged exposure is associated with asthma in children and the formation of potentially more harmful particle pollution. Certain VOCs, like benzene in gasoline, are recognized as carcinogens, sparking considerable interest in finding ways to reduce their presence in homes.

Cummings and Waring identified a broad spectrum of VOC removal rates. The effectiveness of various VOCs varied inconsistently among studies, as did the performance of different plant species. However, they reported that in 196 experiments, plants were capable of purifying an average of 0.062 cubic meters of air per plant per hour, representing approximately 13% of the half cubic meter of air we breathe simultaneously.

To deliver air with significantly reduced pollution, a plant would have to purify a quantity of air several times greater than what a person breathes. However, plants do exhibit some influence, contingent on factors such as their size, leaf shape, and individual characteristics. Therefore, let's delve into studies focusing on a few specific species.

What is the extent of air pollution removal by arrowhead plants?

In a particular study, a small chamber containing an arrowhead plant was injected with enough benzene to elevate the concentration to 250 times the recommended safety limit. The chamber was subsequently sealed for eight days. The arrowhead plant, along with its soil, managed to purify approximately 1% of a cubic foot or just over a third of a liter of air per hour. Interestingly, the purification rate decreased by half when the arrowhead plant was placed in water alone, indicating that a significant portion of the removal process was attributed to the soil and the microorganisms present in it.

How much pollution do Ficus plants remove?

A researcher exposed Ficus plants to formaldehyde at approximately ten times the recommended safety limit for a duration of five hours, along with a relatively safe quantity of toluene and xylene for 24 hours. The experiment took place in a medium-sized chamber (one square meter). Interestingly, various Ficus species performed well in removing the substantial amount of formaldehyde, reaching levels around 10 cubic feet or 300 liters per hour. It remains unclear whether these results are applicable to the significantly lower levels commonly found in households or if they are adequate to mitigate the impact of formaldehyde.

Ficus benjamina, the species employed for formaldehyde removal, exhibited more than a 25-fold increase in formaldehyde removal compared to Mini. While Ficus plants may indeed have the capability to sufficiently reduce formaldehyde levels, it's important to note that plants can incorporate airborne formaldehyde into their metabolisms, possibly due to its simplicity and small size. Placing a few potted Ficus or other plants in the vicinity might contribute to lowering formaldehyde concentrations, particularly during daylight hours when they are actively photosynthesizing.

Utilizing Ficus benghalensis for toluene and xylene removal proved to be less effective, with Mini demonstrating a speed approximately 600 times faster in this regard.

The plants' metabolisms either lacked a purpose for the larger and more complex toluene, xylene, or benzene molecules, or they couldn't eliminate them as rapidly for some other unknown reason.

Plants and particle pollution

Volatile organic compounds (VOCs) represent just one facet of air pollution. Plants have undergone testing to assess their capacity for particle removal, such as those originating from smoke or exhaust. Particle pollution differs from VOC and other gaseous pollutants because particles are generally larger than individual gas molecules and can carry more free radicals and other reactive substances that interact with the body, causing damage.

Unlike formaldehyde, plants cannot absorb particles for utilization. Instead, they eliminate particles by retaining them on the surface of their leaves. Most research on plants and particle removal focuses on the total surface area of a plant's leaves. Other factors, such as leaves with more wax or rougher textures, are also crucial, as they can enhance the plant's ability to retain particles.

A medium-sized fern or spider plant, with their numerous long leaves, provides a substantial surface area for houseplants, totaling around 1,200 square centimeters. In comparison, our lungs possess an internal surface area ranging from 50 to 75 square meters, approximately 500 times larger. Our lungs continuously draw in air using the power of diaphragm muscles, a capacity plants do not possess. Likewise, the dense array of fibers in an air filter offers extensive surface area for trapping particles.

How much pollution do spider plants remove?

In an experiment where spider plants were placed in various occupied rooms for two months, it was observed that the leaves accumulated approximately 2-4 times the weight of particles per area compared to aluminum plates in the same rooms, indicating their efficacy, although not a complete solution. The air quality in these rooms remains unknown, but the maximum amount of particles accumulated on each spider plant was approximately 20 micrograms per square centimeter of leaf. With a spider plant having around 2,000 square centimeters of leaves, this amounts to 50,000 micrograms per plant over the course of two months (for comparison, an aspirin tablet weighs between 300,000-500,000 micrograms).

At a moderate Air Quality Index (AQI) level of 100, a small 250 square foot room contains approximately 9,000 micrograms of inhalable particles (PM10). If the pollution source is external, particle pollution will be exchanged at a maximum rate of once per hour or every fifteen minutes if the window is open.

In comparison to the overall volume of particles in the air, a spider plant doesn't significantly impact the air quality. It contributes just slightly less than the fraction of smaller particles that deposit in your lungs and accounts for around 1% of the particles entering a home from outside. In contrast, the Molekule Air Mini demonstrated the removal of 89% of dust particles in a small chamber within approximately 20 minutes.

Despite their limited impact on air quality, spider plants have aesthetic appeal and are easy to propagate, making them a viable addition to your home. Moreover, as long as the soil is not moldy, they do not worsen air quality. Additionally, there is evidence suggesting that they can remove formaldehyde in one study, although the full data from the authors has not been publicly disclosed.

Analyzing the particle deposition data mentioned above provides insights into how the presence of other plant species might contribute to particle removal.

How much pollution do aloe vera and other succulents remove?

In the study assessing spider plants and their efficacy in removing formaldehyde, it was noted that spider plants outperformed aloe vera in this regard. However, without access to the data, the extent of this difference remains uncertain.

We can make an approximation of the number of particles that might settle on it. A fully grown 6-month old aloe vera plant in standard soil typically has approximately 12 elongated leaves, each spanning about 600 square centimeters. Since the leaves stand upright, let's assume that all 7,200 square centimeters are effective in capturing particles. Plants exhibit considerable variation in their ability to capture particles, with the most efficient ones capable of capturing approximately 75 micrograms per cubic centimeter. For the sake of optimism, let's assume that aloe vera captures particles at this optimal rate.

Even if aloe vera demonstrated exceptional prowess in capturing particles, its ability falls short of making a significant impact on pollution removal. At most, it would eliminate particles at a rate akin to that of a person who is out of breath.

While plants offer various benefits and contribute minimally to air cleaning, they cannot be equated to the efficiency of an air purifier or the effectiveness of proper ventilation.